1. Field of the Invention
This invention relates to methods and apparatus for magnetic surveying, and more particularly to methods and apparatus for precisely locating airborne magnetic field measuring instruments.
2. Description of the Prior Art
Magnetic surveying is, with increasing frequency, done by towing a magnetometer measuring apparatus in one or more "birds" from an airplane. An example of such magnetometer apparatus and method can be found in U.S. Pat. No. 3,418,568, which is incorporated herein by reference.
Presently, in conducting such magnetic surveying, a camera is ordinarily bolted to the frame of the aircraft to photograph the route of the survey for location reference for subsequent data analysis. The rate of which the photographs are taken is determined by the speed of the aircraft, usually so that each photograph will overlap to some degree the last for a continuous photographic record of the area surveyed. Often, however, the camera does not photograph the bird or birds towed by the airplane, it being primarily intended only to locate the position of the airplane along the surveyed path. Because of wind gusts, and other reasons, however, the birds usually do not exactly track or follow the path of the airplane. It can be seen, for example, that even minor corrections in the direction of the airplane may result in large or exaggerated movements of the bird trailing on relatively long tow cables, much like the fast movement presented at the tip of a whip.
Furthermore, because the camera is bolted directly to the frame of the airplane, as small corrections in direction of flight are made by the pilot, the aircraft tilts causing the camera to record the earth's surface beneath the airplane from an angle other than true perpendicular. Thus, in subsequent interpretation of the location of the airplane, for example, in matching the photographs taken with maps, the precise position of the aircraft and, of course, the trailing bird, a necessity for accurate surveying, may be erroneously located.
Another particularly distinct problem encountered in airborne magnetic surveys, especially high precision surveys in which a plurality of magnetic measurements are simultaneously made, for instance, from a number of towed birds to measure the magnetic gradient therebetween, extremely calm air conditions must exist for accurate surveys. It has been estimated, for instance, that in conducting ordinary magnetometer surveying, only one day out of every four, on the average, is suitable for flying and conducting such surveys. Even then, frequently only a minimum acceptable amount of turbulence is present in the early morning hours. Furthermore, the error introduced by air turbulence is compounded in gradient surveys in which far less flying time acceptable for accurate measurements ordinarily exists.
Additional sources of "noise" (i.e. uncertainty in the bird location reflected into the interpretation of the measurement data obtained) is derived from the instantaneous positions of the bird or birds in rolling, horizontal, and vertical movements, which are undetected from the airplane and which change the relative magnitude of the magnetic field measured.
Especially in gradient measurements, the displacement between the birds is a critical factor in obtaining consistent magnetic readings. Very minor variations, for example, of the lower bird with respect to the upper bird generates major noise in the results obtained due to changes in spacing and also in vector direction of the difference measurement.